1. Field of the Invention
The present invention relates to wireless communications and, more particularly, to the desensitization of a wireless receiver.
2. Description of the Related Art
In many cellular and personal communication systems, the power levels transmitted by every mobile unit is under the control of the serving base stations. Power control over the mobile units is done so that the mobile units transmit at the lowest power level necessary to maintain a good quality link on the reverse channel (from the mobile unit to the base station). Power control helps prolong battery life for the mobile unit and reduces the reverse channel signal to noise ratio (S/N) subscriber unit. In certain situations, it may be advantageous to reduce the sensitivity for a wireless receiver at the base station to achieve an appropriate level of transmit power from the mobile unit. For example, where the transmit power of a mobile unit is intentionally kept high to overcome potential interferers, it may be desirable to desensitize the base station, so the base station believes that the mobile is farther away than it really is. As such, the base station will transmit to the mobile unit power control information maintaining a high transmit power for the mobile unit. Desensitization may also be important when the mobile unit is receiving power control information from two or more base stations. In code division multiple access (CDMA) systems where a small coverage area (microcell) is embedded within a larger cell (macrocell) and the mobile unit is in soft handoff (in that the mobile unit is simultaneously communicating with the microcell and the macrocell), the base station receivers of both cells may be sending conflicting power control information to the mobile unit. For example, if the microcell is too sensitive, the microcell base station may transmit power control information to the mobile unit directing the mobile unit to transmit at a low power level. While transmitting at the low power level, the mobile unit may satisfy the microcell base station, but the low power level will be insufficient for the macrocell base station. In response, the base station for the macrocell will transmit power control information directing the mobile unit to transmit at a higher power level which is not acceptable to the microcell base station. Therefore, it is necessary to desensitize the microcell so that the transmitted power level of the mobile unit at the handoff boundary is sufficiently high to both the microcell as well as to the macrocell, thereby achieving an appropriate sensitivity balance between the different base stations.
FIG. 1 shows a manner in which desensitization is implemented in the front-end of a wireless receiver 10. The receiver 10 includes an attenuator 12 or other lossy device which is connected prior to the input of a low noise amplifier 14. The attenuator 12 can be adjustable to provide variable desensitization levels. The attenuator 12 provides desensitization by reducing or attenuating the signal power level while the noise power level remains at the same level. Depending on the desired level of desensitization, the amount of attenuation of the received signal is set accordingly. By reducing the signal power level while maintaining the noise level, the S/N ratio for the receiver is reduced and thereby the sensitivity of the receiver is reduced. The disadvantage of setting the sensitivity of the receiver in this manner, however, is that variable attenuator is on the receive path 15 and always incurs some insertion loss. This loss can be as much as a few decibels (dB) and may sometimes be a significant contributor to the overall noise figure of the receiver 10. Accordingly, a need exists for a desensitization system for setting the sensitivity of a wireless receiver that reduces the drawbacks of previous systems.